Heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different material. This technology is often used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. In particular, Group-III nitrides on silicon heteroepitaxy have recently demonstrated to be a viable alternative for growing high-quality group-III nitride films for optoelectronic, electronic, and surface acoustic wave device applications. Silicon has become the substrate of choice owing to the availability of larger size (up to 12 inch in diameter), low cost, and excellent crystal quality of Si substrates, as well as possessing excellent material properties such as doping properties (amphoteric type and high carrier concentration), cleavability, good thermal conductivity (about 3 times larger than that of sapphire), and mature processing techniques. These advantages of Si substrates provide many novel applications of group-III nitride materials, including the potential integration of GaN and Si technologies.